One of the primary applications of geotechnical engineering is in the design of foundations for offshore oil and gas platforms. One well known type of offshore platform comprises a welded steel framework, known as a "jacket structure", which rests on the floor of the body of water and supports the platform's drilling and producing facilities. Typically, the jacket structure is attached to the floor of the body of water by a plurality of elongated piles which are driven a predetermined distance into the underlying soils and are then grouted or otherwise attached to the jacket structure. These piles must be capable of transferring all axial and lateral loads acting on the platform to the underlying soils.
In recent years, a new type of offshore platform known as a "tension leg platform" or "TLP" has been developed, primarily for use in deep waters. Basically, a TLP comprises a buoyant hull which is attached to one or more foundation units on the floor of the body of water by a plurality of substantially vertical tethers. The length of the tethers is adjusted to maintain the buoyant hull at a greater draft than it would have if it were unrestrained. The resulting excess buoyancy exerts an upward tensile load on the tethers which must be resisted by the foundation units. Typically, the foundation units are attached to the floor of the body of water by a plurality of elongated piles which must be capable of withstanding these tensile loads for the life of the structure.
As will be evident from the above, the design of a pile foundation for an offshore platform can be of crucial importance. Unfortunately, the interaction between a steel foundation pile and the surrounding soils is very complex and is not well understood. Consequently, geotechnical engineers have expended substantial efforts in developing new tools to aid in understanding the various factors which influence the soil/pile interaction. A better understanding of these factors will help to ensure that pile foundations for future offshore platforms are safe and reliable.
Two of such geotechnical tools are described in Boggess, et al., "Advanced In-Situ Instruments for Studying the Behavior of Cyclically Loaded Friction Piles" presented at the 1983 American Society of Civil Engineers (ASCE) Annual Convention in Houston, Texas. Both of the tools described by Boggess, et al. are used in situ to simulate the behavior of short segments of a foundation pile. After being inserted into the soils, the tools measure local friction, local displacement, pore water pressure, and total lateral pressure under a variety of loading conditions.
One of these tools, known as the "X-probe", is similar in external configuration to a conventional cone penetrometer. It has a diameter of 1.72 inches, a length of 56.5 inches, and is capable of being deployed by existing cone penetrometer equipment. Its primary purpose is for routine site investigations at the proposed site of offshore oil and gas facilities.
The other tool described by Boggess et al. is 3 inches in diameter and has a total length of approximately 16 feet. It is primarily intended as a research tool for investigating the soil/pile interaction. It comprises a cutting shoe section which can simulate either an open-end or a closed-end pile and an instrument section which houses the various instruments used to measure the soil/pile interaction parameters. During loading, a slip joint permits relative axial movement between the instrument section and the cutting shoe section which serves to anchor the tool in the surrounding soils. This allows direct measurement of the shear-displacement characteristics of the soil/pile interaction.
Additional information on these two geotechnical tools can be found in Bogard, et al., "Three Years' Experience With Model Pile Segment Tool Tests", OTC 4848, presented at the 1985 Offshore Technology Conference in Houston, Tex.
Other in situ geotechnical tools have also been developed. See, for example, Coop, et al., "Field Studies of an Instrumented Model Pile in Clay", Geotechnique, December 1989, pp. 679-696 which describes a model pile tool developed at Oxford University. See also, Morrison, M. J., "The Piezo-Lateral Stress (PLS) Cell", Chapter 3 from Phd. thesis submitted to Massachusetts Institute of Technology, 1984, which describes a model pile tool developed at M.I.T.
One disadvantage of most prior model pile tools is that they are generally designed to be installed by pushing rather than by pile driving. Installation by pushing creates a different stress state in the surrounding soils than installation by pile driving. Accordingly, in order to accurately model the behavior of a full scale driven foundation pile, the model pile tool should be installed in the soils by pile driving rather than by pushing.
It should be noted that the Boggess, et al. paper discussed above indicates that both the X-probe and the 3-inch diameter model pile segment are designed to be inserted by driving or pushing. However, in actual practice, these tools were not rugged enough to be installed by driving in any but the softest of soils. In some types of soils, such as dense sands, pile driving results in very high impact loads and accelerations. None of the prior model pile tools was capable of withstanding these loads and accelerations.
Another disadvantage of prior model pile tools is their inability to measure dynamic loads and accelerations during installation by pile driving. Measurement of dynamic loads and accelerations would permit calculation of the dynamic skin friction between the pile and the surrounding soils. Dynamic skin friction, in combination with static skin friction, may be used to establish the damping parameters required for analyses of pile driving performance.
Accordingly, a need exists for an in situ model pile tool which is capable of being installed in dense soils by pile driving and which can be used to measure dynamic loads and accelerations during installation.